Fix return-from-declaration when navigating to an unsaved file

[mislav]

Fixes the following bug:

• Create a new file, but don't save it.
• Use go-to-declaration to jump to the definition of the word under cursor.
• Use return-from-declaration to jump back.

Expected: the active text editor focus returns to unsaved file.

Actual: a new blank file tab gets opened.

[thedaniel]

working well for me locally, anything else from @atom/feedback ?